1. Field of the Invention
The present invention relates generally to a combined inorganic, dielectric grid polarizer and diffraction grating to polarize and further control light, such as by reducing zero order back reflection.
2. Related Art
Diffraction gratings are a periodic structure of dielectric material with a period (p) greater than half the wavelength (λ) of incident light, or p≧λ/2. The diffraction grating scatters the incident light at discrete angles or directions in accordance with mλ=p sin θ, where m is the order and θ is the angle with respect to normal from the diffraction grating. Thus, different wavelengths are reflected or scattered at different angles.
Wire grid polarizers are a periodic structure of conductive elements with a length greater than the wavelength and a period less than half the wavelength of the incident light, or p≧λ/2. Wire grid polarizers have been proven to be effective for visible light (˜300-700 nm, or ˜0.3-0.7 microns or μm) and their use demonstrated as polarizers and beam splitters in optical imaging systems.
Various imaging systems, such as projection displays, using liquid crystal spatial light modulators, such as liquid crystal on silicon (LCOS), have been proposed that utilize polarizers, such as wire grid polarizers. For example, see U.S. Pat. Nos. 6,234,634 and 6,447,120. Such polarizers, however, can also back reflect a portion of the incident light resulting in a ghost image. One solution has been to tilt or angle the polarizer to direct the back reflection out of or away from the optical path. Tilting the polarizer, however, can take-up valuable space in a compact design, and can cause unwanted astigmatism. In addition, the conductive metal of the wires can absorb light.
Various types of polarizers or polarizing beam splitters (PBS) have been developed for polarizing light, or separating orthogonal polarization orientations of light. A MacNeille PBS is based upon achieving Brewster's angle behavior at the thin film interface along the diagonal of the high refractive index cube in which it is constructed. Such MacNeille PBSs generate no astigmatism, but have a narrow acceptance angle, and have significant cost and weight.
Another polarizing film includes hundreds of layers of polymer material stretched to make the films birefringent. Such stretched films have relatively high transmission contrast, but not reflection contrast. In addition, polymer materials are organic and not as capable of withstanding higher temperatures or higher energy flux. For example, see Vikuiti™ polarizing films by 3M.
Composite wire-grid polarizers have been proposed in which the wires include alternating layers of dielectric and conductive layers. For example, see U.S. Pat. Nos. 6,532,111; 6,665,119 and 6,788,461. Such polarizers, however, still have conductive materials.